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Stockholders' Equity - Narrative (Details)
6 Months Ended
Jun. 30, 2018
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Outstanding options that are exercisable (shares) 1,400,000
Outstanding options that are exercisable, weighted average exercise price (USD per share) | $ / shares $ 41.38
Employee Stock Purchase Plan  
Employee Stock Purchase Plan  
Share purchase price as percent of market price (as a percent) 85.00%
Shares available for future purchases (shares) 65,007